A macromolecular complex of eight amino acyl tRNA synthetases (amino acid tRNA ligases E.C.6.1.1) has been isolated from lactating rat mammary gland, and a similar enzyme complex from rat mammary ascites tumor cells (Adenocarcinoma 13762). Using affinity chromatography, an attempt will be made to correlate the activity of the enzyme preparations with the several types of 100A diameter particles observed in our enzyme preparations on electron microscopy. We also plan to initiate a study of the partial reactions concerned in peptide chain elongation, using an elongation factor preparations complexed with synthetases.